Carbon steel corrosion by iron oxidising and sulphate reducing bacteria in a freshwater cooling system

Microbiologically influenced corrosion of carbon steel has been investigated. Carbon steel coupons were exposed online in the cooling water system of a nuclear test reactor to assess the microbial growth on the coupons and the corrosion phenomena. Iron bacteria, sulphate reducing bacteria (SRB) and culturable aerobic heterotrophic bacteria (CAHB) were monitored both on the coupons and in the cooling water. Corrosion rate was assayed by weight loss method and corrosion products analysed by XRD and Mossbauer spectroscopy. Extensive tuberculation of carbon steel coupons was noticed. SEM pictures revealed the presence of ensheathed filamentous iron bacteria encrusted with corrosion products. Beneath the tubercles significant pitting and SRB induced corrosion in the form of concentric rings was observed. From the phase analysis, the following compounds were found to be present: gamma-Fe2O3, Fe2PO5, FePS3, Fe(PO3)(3) and BaFeO3-x. From the present study it is inferred that iron bacteria (Leptothrix sp.) and SRB (Desulfovibrio sp.) are responsible for the corrosion of carbon steel. The role of these bacteria in influencing the corrosion of carbon steel is highlighted. (C) 2000 Elsevier Science Ltd. All rights reserved.